The present invention generally relates to optical display devices and more particularly to an optical display device that uses a light valve such as an optical projector for projecting an image on a screen by means of a spatially modulated optical beam produced by a light valve. Further, the present invention relates also to a direct-view-type liquid crystal display device.
An optical projector is a device for projecting images on a screen by means of a spatially modulated optical beam produced by a light valve. In order to improve the visibility of the projected image, intensive efforts are being made to increase the luminance of the optical projectors. In conventional optical projectors, a luminance of about 600 lumens has been required on the screen. Recently, however, there are demands for a luminance of as much as 1000-1200 lumens on the screen.
In order to meet the demand, recent optical projectors tend to use a high-power metal-halide lamp of 350 W or more as compared with conventional optical projectors that typically use a metal-halide lamp of 250 W.
On the other hand, the use of such a high-power optical source causes a problem of severe heating of a polarizer used in the optical projector in combination with the light valve for producing a linearly polarized optical beam. For example, the polarizer surface is exposed to a luminance of as much as 200 million luxes in such high-power optical projectors. This value of luminance is twice as large as the luminance in a conventional optical projector.
A conventional optical projector typically has a construction that includes a metal-halide lamp as an optical source for producing an optical beam, a polarizer provided in a path of the optical beam for polarizing the incident optical beam to produce a linearly polarized optical beam, a light valve disposed in an optical path of the linearly polarized optical beam for a spatial modulation thereof, an analyzer disposed in an optical path of the spatially modulated optical beam to complete the optical spatial modulation, and a projection lens that projects the output optical beam exiting the analyzer on a screen. The polarizer typically includes dye molecules or iodine molecules dispersed in a polymer matrix and absorbs polarization components in the optical beam except for the component that has a polarization plane perpendicular to an absorption axis of the polarizer.
The optical beam components thus absorbed by the polarizer naturally induce a temperature rise in the polarizer itself, and because of this, conventional optical projectors have used a bulky cooling mechanism for cooling the polarizer as an indispensable element thereof. In a typical example, such a cooling mechanism is used for suppressing the temperature of the polarizer below about 70.degree. C. for the polarizers that use dye molecules. In the case of iodine polarizers, the temperature has to be suppressed below about 60.degree. C. As already noted, the luminance reaches as much as 200 million luxes at the polarizer surface when the metal-halide lamp of 350 W is used in place of the metal-halide lamp of 250 W. Thus, the temperature of the polarizer easily exceeds the foregoing tolerable limit as a result of the absorption of about one-half the energy of the foregoing 200 million luxes when the polarizer is not properly cooled.
Thus, the use of a cooling mechanism has been indispensable in conventional optical projectors for cooling the polarizer. However, such a use of the cooling mechanism inevitably increases the size as well as the cost of the optical projector.